The present invention relates generally to voice communications systems and, more particularly, to an apparatus for performing voice-band spectrum inversion using digital processing.
A typical cordless telephone system includes a radio frequency (RF) transmitter/receiver (transceiver) both in the handset and in the base station for communication of the voice-band signal therebetween. It has long been recognized that such RF communication between the handset and base station is subject to easy interception using other cordless telephones, scanners or baby monitors. Given the relatively small number of RF carrier frequencies available for use in cordless telephone systems of this type, it is not surprising that a concern for message security exists, even among residential users.
Because message security has long been recognized as a shortcoming of cordless telephone systems, there have been many types of solutions proposed. Historically, most of these solutions have been implemented with analog circuitry and, in particular, using custom linear integrated circuit devices. A spread spectrum approach, e.g. , frequency hopping, is considerably more complex than is warranted to provide the level of telephone security needed in most consumer applications.
There is a class of solutions to the problem of cordless telephone message security which does not lend itself to implementation by analog circuitry and must, in fact, be implemented using digital techniques. Digital signal processing circuits are becoming increasingly prevalent in consumer electronic products, and their use in this type of application generally tends to provide simpler, more efficient and more reliable solutions. Furthermore, where a digital signal processor may already exist in the handset of a cordless telephone for the control of other supervisory and signaling functions, sharing its processing power within a message security sub-system my help to provide an extremely inexpensive solution to a long-felt problem.
In view of the above, it is clear that there exists a need to develop an improved apparatus and method for providing message security in cordless telephone systems than is currently known in the art. Furthermore, where such improved apparatus and method employ digital techniques, enhanced operational features may be expected to follow therefrom.